1. Field of the Invention
The invention relates to a high-accuracy flowmeter for testing and calibrating flowmeters and the like, and more particularly to a positive displacement flowmeter which may include a cylinder, a piston within the cylinder which makes a clearance seal with the inside of the cylinder, an encoder associated with the piston, and a control valve for controlling the supply of a fluid flow to be measured. Other features are also disclosed.
2. Description of Related Art
A known type of positive displacement flow calibrator measures the time a soap film bubble takes to move a known distance in a tube. These devices are position-sensitive, must be kept filled with solution, are unpredictable (double bubbles, broken bubbles, etc.) and may inject moisture into the pump being calibrated. Accuracy can be compromised by change of bubble shape during a reading, particularly at high flow rates. Another important disadvantage of these bubble flowmeters is that many seconds may be required to obtain a reading at full accuracy, particularly under low flow conditions, because their sensors are a fixed distance apart. There is no possibility of obtaining intermediate readings. Also, remote or automatic operation is impossible, since the flowmeter must be maintained and bubble quality must be observed.
Other displacement flowmeters use piston-and-cylinder arrangements of various types, which inherently create friction and are unusable at low-pressure or low-flow levels, even if Hg or oil seals are used. Liquid seals may also contaminate the system being measured.
For example, U.S. Pat. No. 2,320,447 to Raymond discloses a volume flowmeter particularly intended for high-liquid-pressure applications, in which a fluid flow is diverted from a main flow line into a chamber containing a piston. The force of the fluid moves the piston along the chamber. The rate of flow is then calculated by measuring piston displacement during a given predetermined time interval. Raymond describes a series of sealing grooves in his piston which apparently contain sealing elements or a fluid lubricant and are said to prevent leakage around the piston. A significant disadvantage of Raymond's device is that contact between the piston and cylinder as disclosed therein causes excessive friction in low-pressure and/or low-flow measurements.
U.S. Pat. No. 2,892,346 to Sargent discloses a complicated volume flowmeter in which a piston is moved, not by the fluid flow being measured, but by a servomotor, along a chamber in response to a measured pressure difference between the pressure in the chamber and ambient pressure, so as to maintain such pressure difference at a minimum.
U.S. Pat. No. 4,307,601 to Jackson describes a flow calibrator which employs a cylinder containing a mercury-sealed piston which is attached to a counterweight by a tape which is trained over a pulley. The pulley rotates an encoder and pulses from the encoder are counted at regular predetermined time intervals to determine the movement of the piston for each of those time intervals. Jackson's device must be started and stopped by pushbuttons, which prevents automatic operation.
A significant disadvantage of both Raymond and Jackson is that piston displacement must be measured over a predetermined time interval. Intermediate flow readings during such time interval are not available. Also, precision is limited, since Raymond's readings come from a visual scale and even Jackson's digital encoder pulses come too slowly to obtain good precision.
U.S. Pat. No. 3,125,879 to Porter and U.S. Pat. No. 2,927,829 to Porter disclose other flowmeters which include pistons sealed by liquid, preferably mercury.
Also known to the art is a flowmeter with a horizontal shuttling piston for measuring a fluid flow rate. Such prior flowmeters measure the number of complete shuttles in a predetermined length of time.
The disclosures of these and other prior art references mentioned herein are incorporated by reference.